1. Field of the Invention
The present invention relates generally to complex, composite structures and an apparatus and method for fabricating such complex structures from composite fiber/resin. More particularly, the present invention relates to an apparatus and method for fabricating complex structures with a plurality of helical and/or reverse helical components, such as trusses or cylinders.
2. Related Art
Complex truss structures have been described in U.S. Pat. No. 5,921,048 that can have enhanced load bearing capacity per unit weight. Such structures can have complex configurations, including for example, a plurality of helical components formed around a longitudinal axis in opposite directions. Each helical component can include a plurality of sequential straight segments coupled end-to-end in a helical configuration. Each helical component can include three segments forming a single, complete rotation about the axis, such that the helical component has a triangular cross-sectional shape formed by the straight segments when viewed along the axis.
The plurality of helical components can include both 1) spaced-apart helical components formed around the axis in one direction or with one angular orientation, and 2) spaced-apart reverse helical components formed around the axis in another direction or with an opposing angular orientation. The straight segments of the helical and reverse helical components can form a triangular cross-sectional shape when viewed along the axis. In addition, the helical components further can include 3) spaced-apart rotated helical components, and 4) spaced-apart rotated reverse helical components, that are similar to the respective helical and reverse helical components, but rotated about the axis with respect to the helical and reverse helical components. Thus, the helical and reverse helical components form a first triangular shape, while the rotated helical and rotated reverse helical components form a second triangular shape concentric with, but rotated with respect to, the first triangular shape, to form a six-pointed star shape.
The various helical components form a basic repeating pattern along the length of the structure. In addition, the various helical components intersect one another at internal and external nodes, with the external nodes being spaced further from the axis than the internal nodes. For example, the helical and reverse helical components intersect at internal and external nodes. The structure further can include axial components that extend along the length of the structure parallel with the axis. Such axial components can intersect the helical components, including for example, at the internal and/or external nodes.
It is desirable to form such structure from composite materials to reduce weight and increase strength. In addition, it is desirable to form the helical and axial components from continuous fibers to further maximize the strength of the structure. Thus, the fibers are traversing along the structure at various angles. As stated above, such structures have shown unexpected stiffness, and strength or load bearing capacity per unit weight.
The fabrication of such structures, however, has proven to be very difficult. Wide-spread application of such structures has been frustrated by the inability to quickly, easily, and/or inexpensively manufacture such structure. It will be appreciated that such structures have complex geometries or configurations. It also will be appreciated that such complex geometries have proven ill suited for conventional manufacturing techniques.
Various manufacturing processes exist for composite fiber/resin. For example, in a pultrusion process, the fiber and resin is extruded and pulled through a die having the desired, continuous, cross-sectional shape. As another example, braiding processes overlap fibers into a sock or sleeve configuration in a continuous, closed layer. Such sleeves can be formed or disposed over a mandrel or around a die. As another example, mandrel techniques wind fiber about a solid model or mandrel with a continuous, solid outer surface having the desired configuration about which the fibers are disposed. After the fiber has been impregnated with resin, and the resin cured to form a rigid structure about the mandrel, the mandrel can be removed to leave the rigid structure.
None of these existing technologies appears suited for continuous or volume manufacturing of such complex, three-dimensional structures described above. For example, it will be appreciated that the complex, three-dimensional nature of the structure, with the straight segments extending through the structure between external nodes, makes any mandrel shaped as the structure difficult to remove from the structure itself. Similarly, it will be appreciated that the complex, three-dimensional structure has a varying cross-sectional shape, and discontinuous or open surface structure, which is ill-suited for conventional pultrusion techniques. As another example, it is unclear, how braiding techniques could be used to fabricate more complex and open structures, such as those described above.
In addition, the intersections of the various helical components at the nodes have also proven problematic. It will be appreciated that as the various fibers intersect, gaps can be formed between the fibers which can reduce the strength of the structure by as much as 90 percent.